Drive Train Axles Bearings problem
1997 Dodge Truck V8 Four Wheel Drive Automatic 175000 miles

I have dodge ram 1500 4x4 5.9l truck. I hear sound coming from rear differential if i drive between 42 and 50 miles p/h. I replaced all the bearings in rear differential. The sounds comes if i push on gas pedal only, if i realize the sounds disappears. What could be wrong, my mechanic doesn't known too.